A standard IC CMOS crystal oscillator in a Pierce oscillator circuit configuration is illustrated in FIG. 1. A single inverter stage I1 is coupled between an oscillator input OSC IN and an oscillator output OSC OUT and provides amplified inverted output signals of low and high potential levels in response to feedback input signals of high and low potential levels. The inverter stage I1 includes a PMOS pullup transistor P1 having a primary current path through source and drain nodes coupled between a high potential power rail V.sub.CC and the oscillator output OSC OUT for sourcing current to the oscillator output. An NMOS pulldown transistor N1 has a primary current path through drain and source nodes coupled between the oscillator output OSC OUT and a low potential power rail GND for sinking current from the oscillator output. The inverter stage I1 amplifies the signals with a specified gain A.sub.N.
An oscillator feedback circuit is coupled between the oscillator output OSC OUT and the oscillator input OSC IN for causing oscillation by the CMOS oscillator circuit. The oscillator feedback circuit incorporates an oscillator crystal XTAL having a selected operating frequency within a narrow frequency band at an amplifying gain A.sub.N between operative minimum gain A.sub.MIN and maximum gain A.sub.MAX limits characteristic of the oscillator crystal XTAL. The oscillator circuit generally incorporates bias resistors R1,R2 and bias capacitors C1,C2 in the feedback circuit biasing network.
For oscillation to occur in the Pierce oscillator, two criteria must be met. First the gain A.sub.N of the active inverter stage I1 must be great enough to drive the crystal XTAL and biasing network to cause oscillation. Generally the gain around the oscillator feedback loop and active inverter stage must be greater than 1. To achieve this the gain A.sub.N of the inverter stage I1 must be sufficient to compensate for the gain loss through the impedance of the crystal XTAL and biasing network and generally at a minimum gain A.sub.MIN characteristic of the crystal XTAL for sustaining oscillation. If the gain is too great, i.e. greater than a maximum gain A.sub.MAX also characteristic of the crystal, then the crystal oscillator may not start. Thus, the Pierce crystal oscillator will only operate within a selected gain range or selected gain limits A.sub.MIN, A.sub.MAX.
The second criterion is that the total phase shift around the oscillator feedback loop must be 360.degree.. The total phase shift is ideally achieved through the 180.degree. phase shift across active inverter stage I1 and the 180.degree. phase shift across the crystal XTAL. The phase shift across the crystal XTAL and biasing network tends to compensate for the additional phase shift caused by the propagation delay through inverter I1.
A disadvantage of the conventional Pierce crystal oscillator integrated circuit configuration is the limited frequency band of operation of the oscillator. The crystal oscillator is limited to the narrow band operating frequency of a particular crystal or a limited number of crystals with operative gain ranges A.sub.MIN, A.sub.MAX encompassing the fixed gain A.sub.N of the particular inverter stage I1.